Hi, good day! I'm John Dev a Windows user like you and I'll be happy to assist you today. I know this has been difficult for you, rest assured, I'm going to do my best to help you.
Please try to download and run the .NET Framework Repair Tool.
https://www.microsoft.com/en-us/download/details.aspx?id=30135
After that, restart your computer and install the .NET 3.5 framework again.
I look forward to receiving your update and I'm willing to collaborate with you till we resolve the issue. Do keep me informed.
Kind regards,
John DeV
Independent Advisor